Wafer-Level Packaging (WLP) refers to the technology of packaging an integrated circuit at wafer level, instead of the traditional process of assembling the package of each individual unit after wafer dicing. In the wafer-level packaging, processes such as, for example, wafer fabrication, packaging, test, and burn-in at wafer level can be integrated, thereby reducing costs and increasing throughput.
In the wafer-level packaging, a plurality of dies are formed on a wafer, and then a redistribution layer is formed over the dies. The redistribution layer includes a dielectric layer, a plurality of ball-pad arrays on the dielectric layer, and a redistributed metal line in the dielectric layer that are coupled to individual ball-pads of the ball-pad arrays. Each ball-pad array is arranged over a corresponding die, and the ball pads in an array are coupled to corresponding bond-pad of a die by the redistributed metal line. After forming the redistribution layer on the wafer, solder paste is deposited onto the individual ball pads. The solder paste is then reflowed to form small solder balls on the ball pads. After forming the solder balls, the wafer is cut into pieces.
FIG. 1 is a cross-sectional view showing a conventional wafer level package. Referring to FIG. 1, the wafer level package 101 includes a silicon substrate 111, a passivation layer 131, an aluminum (Al) pad 141, a redistributed metal line 161, a solder ball 181, a first polymer layer 151, and a second polymer layer 171. The passivation layer 131 and the Al pad 141 are formed on the silicon substrate 111. The first polymer layer 151 is formed on the passivation layer 131 and the Al pad 141. A seed metal layer 121 comprising, for example, Titanium (Ti) and Copper (Cu), is deposited on the first polymer layer 151. Then, a photoresist layer (not shown) is formed on the first polymer layer 151, and the photoresist layer is patterned. A metal such as, for example, a copper (Cu) and nickel (Ni), is deposited over the seed metal layer 121 to form a metal line using, for example, electroplating. The photoresist layer and the seed metal layer 121 below the photoresist layer are removed by etching. The second polymer layer 171 is formed on the first polymer layer 151 and the redistributed metal line 161. An opening is formed to expose an end portion of the redistributed metal line 161 positioned opposite the Al pad 141. Then, the opening is filled with a solder ball 181. The second polymer layer 171 covers the redistributed metal line 161 except for the opening. As can be seen from FIG. 1, the entire redistributed metal line 161 makes contact with the first or second polymer layers 151, 171.
Due to differences in expansion and contraction characteristics between metal and polymer, delamination between the redistributed metal line 161 and the first polymer layer 151 is prone to occur, and is so, the connection between the pad and the solder ball 181 using the metal line 161 is opened, causing the package to fail.